1. Field of the Invention
This invention relates to solid electrolyte fuel cells, and more particularly, provides an improved generator system comprised of such cells.
2. Description of the Prior Art
High temperature solid electrolyte fuel cells such at those disclosed by the above-referenced application convert chemical energy into direct current electrical energy, typically at temperatures above 700.degree. C. This temperature range is required to render the solid electrolyte sufficiently conductive for low power losses due to ohmic heating. With such cells, expensive electrode catalysts and refined fuels are not required. For example, carbon monoxide-hydrogen fuel mixtures can be used directly without conversion. Stabilized zirconia is a prime electrolyte candidate and is used in thin layers on ceramic tubular support structures. The support tubes for thin film high temperature solid oxide electrolyte cells are generally also made of stabilized zirconia and serve as ducts for one of the reactants, fuel or oxidant. This requires porosity in the support tubes.
A problem arises in the construction of generators because the fuel and oxidant, such as air, must be preheated to temperatures that require high temperature heat exchangers, such as those comprised of ceramics, a technology that is for present purposes economically unavailable. In fuel cells of the prior art, such as exemplified by the above-referenced application, fuel consumption is not complete and 5 to 15 percent will remain in the anode exhaust. Similarly, an oxidant, such as air, which typically also functions as a coolant, is depleted in the fuel cells, although the oxygen depletion of air is low. The depleted fuel is not utilized to its full capacity in production of electricity. The generator utilizes the non-electrochemical combustion reaction between the depleted fuel and the depleted oxidant, as well as the sensible heat contained in the reaction products, to provide preheating as necessary for the electrochemical reaction. Thus, the generator incorporates a high temperature preheater which eliminates the need for a separate high temperature heat exchanger.
Unfortunately, the use of such a preheater in the design set forth in the above-described application relies on cross-flow heat transfer. As a result, because the heat is generated in one axis and flows in a direction which is perpendicular to that axis, there are temperature variations within the preheater section which can result in different electrochemical reaction rates within the fuel cells and thereby result in inefficiencies within the generator itself.
What is needed is an apparatus for preheating within the fuel cell generator which will avoid temperature variations within the preheater section. Recognizing that the process conveyance tubes are of ceramic material, this apparatus must accomplish its preheating function in a manner which does not require unusual fabrication requirements of ceramic materials.